Specific antibody responses against Neospora caninum recombinant rNcGRA7, rNcSAG4, rNcBSR4 and rNcSRS9 proteins are correlated with virulence in mice.

The intraspecific diversity of Neospora caninum is a determinant for in vivo parasite virulence and in vitro parasite behaviour. The relationship between isolate virulence and specific antibody responses against key parasite proteins has not been well characterized. The response kinetics and the differences in specific anti-rNcGRA7, -rNcSAG4, -rNcBSR4 and -rNcSRS9 antibody levels were analysed by recombinant protein-based ELISA in groups of mice inoculated with 10 different N. caninum isolates that differ in their virulence. The majority of the virulence parameters analysed correlated with the specific antibody levels against the 4 recombinant proteins. The antibodies developed against the highly immunogenic protein NcGRA7 were significantly higher in mice inoculated with high virulence isolates than in those inoculated with low-to-moderate virulence isolates in both non-pregnant and pregnant mouse models. Moreover, these levels were correlated with the anti-N. caninum IgG1 and IgG2a responses and the in vitro tachyzoite yield at 56 h. The antibodies directed against the bradyzoite-specific proteins were not detected in a non-pregnant mouse model. However, some seropositive mice were found in groups inoculated with high virulence isolates in a pregnant mouse model. NcGRA7 and NcSAG4 are proteins clearly correlated with virulence, and to a lesser extent NcBSR4 and NcSRS9 proteins. Moreover, antibodies to bradyzoite-specific proteins appear to also be related to virulence in mice. Further analyses should be performed in order to verify the usefulness of these proteins as predictive markers for virulence in an experimental bovine model of neosporosis.